Easy To Build Underwater Boards
by Pat Patteson - Molalla, Oregon - USA

I’ve been interested in creating “Foil” Like shapes for Dagger and Center Boards for a long time, and have been reading the recent articles on calculating and building “Foils” with great interest. While the math is intriguing, I am more interested in Easy To Build “Foils Shaped” underwater boards. Befor I go too far here, I want to make it clear what I am building are “Foil Like” boards and Not NACA Foils. Just more “Streamlined” shapes. Most of the little boats we build will benefit very little or none at all from high performance, high aspect ratio NACA Foils anyway, but we all try to give our boards some sort of “Streamlined” shape.

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We all try to give our boards some sort of “Streamlined” shape.

I am using the term “Foil” very loosely here and make no pretence that what I build is a true “NACA” foil, or “Foil” of any sort. Just something that Looks good, works well and is fairly easy to build. Although I think one could use some of the technical information in the previous “Foil” articles and this technique to come pretty darn Close to getting a real “Foil” shape.

For those that are interested in learning more about Real “Foils” and “Underwater Board Shapes” from “Experts”, here are some articles I found just so you know that I Know the difference.

Here’s an old (1997) article on NACA foils by Craig O’Donnell.

Jim Michalak” “Underwater Board Shape

Michalak: “Sail Area Math

The fact is, low aspect ratio, “Flat Plate” boards work well on many of our, small, relatively slow boats. And they are used on large “Working Type” sailboats as well. Low aspect, flat boards work over greater range of angles, speeds and conditions and do not stall as easily as higher aspect deep boards, even the NACA “Foils”. High aspect NACA Boards have a definite advantage and are required on many high speed, mono-hull and most multi-hull racing sailboats. The original Hobie Cats are an exception to that rule. The old Hobies have asymmetrical hulls that have a shape kinda like an underwater wing to help keep the boat from sliding sideways. The lack of any underwater boards make them great “Beach Boats”, but even Hobie has now added boards to their newer boats to make them faster and point higher. I still like running my “Hobie 16” up on a beach at high speed, with no concern for Any Boards.

My point is not to discuss the advantages and disadvantages of underwater board aspect ratios, but to explain a Simple method of building boards I “Discovered” many years ago. I say, “discovered” because I had not seen this method used until I saw it used for the Ballasted Daggerboard on Phil Bolger’s “Single Handed Schooner” or as it is also called the “His and Hers Schooner”. The “His and Hers Schooner” along with drawings of the board are shown in his “Boats With An Open Mind” and the Schooner is highlighted here in Chuck’s “Projects”.

I built a Bolger “Elegant Punt” some 25 years ago and decided I wanted to try something other than a flat, plywood leeboard, so did some experimenting with thin plywood skins bent around a “Long Ways” Stringer.

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The board is pretty simple and is built with skins and stringer(s) much like the way airplane wings used to be built.

The board is pretty simple and is built with skins and stringer(s) much like the way airplane wings used to be built, but without the many ribs inside a wing that give the wing its precise shape. I found a thin plywood “Skins” will naturally take a “Foil-Like” shape if bent around a single stringer that is placed slightly forward of the middle of the board.

The construction sounds and is a little more complicated than most standard, single piece plywood, laminated plywood or glued up, laminated solid wood boards. But it is not That complicated and produces a very strong, lightweight board that doesn’t require any “Shaping” to get to that shape most of us feel needs to be in a board. The total cost of material is probably not much more than a conventionally built board and may be less for larger boards.

The board is mostly hollow, so any ballast needed to sink the board can be added when the board is being built, rather than having to cut a hole for lead in a finished board, add lead, and then recover the cutout. As in the board for Bolger’s Schooner, substantial additional ballast can easily be easily added to provide stability, too. Building a hollow board like this is Fun too, and isn’t Fun much of the reason we build these little boats anyway?

Most conventional boards involve creating a blank of wood and then shaping that blank until it Looks like what we think a board should look like. This hollow board technique is Certainly quicker and easier than stacking and gluing multiple layers of plywood or solid, then trying to plane and sand them all to an exact NACA Foil shape. Lots of planing and sanding is Not My idea of Fun. I guess I’m a lazy boat builder, but I enjoy other parts of building more.

This hollow board technique works best for rectangular boards, but can be used to build tapered boards as well. You just have to figure out where to put the stringer or stringers to give the final shape you want.

For the boards I have built, I use the same general overall dimensions I would use if the boards were to be built a conventional way, but I figure the board Will be a little thicker in cross section. That’s part of the reason this type of board is so strong. I also build the boards before I build any box for them as these boards are a “Cut and fit, Work in progress”, ‘till they are done, so I’m not always sure of the Exact finished size.

The boards I have built so far have been relatively small, “Elegant Punt” size, about 4’ long, a foot or so, wide and 1-1/2” thick.

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The boards I have built so far have been relatively small.

I first cut a “Nose Piece” or leading edge piece from solid wood. The “Nose Piece” is just a long rectangular piece of wood slightly thinner than the overall thickness of the board and slightly shorter than the overall length of the board. I cut a rabbet in the back edges of that piece about 3/4” front to back and slightly deeper than the thickness of the plywood I will use for the skins. That rabbet will take the front edge of the plywood skin and is cut slighter deeper so the joint can be sanded flush. So far I’ve used 1/4” plywood for skins for all my hollow boards so the rabbets have been about 3/4” front to back and slightly deeper than 1/4”.

Depending on the type of board, (leeboard, dagger board or centerboard), I cut a solid piece of wood that will fit between the skins at the top of the board to act as a solid spacer block and to reinforce a pivot pin hole if needed. The solid piece also makes a solid place to attach a top “Handle”. As is the case of my Michalak style leeboard, I attached a long handle to swinging the board up and down.

Since the top part of the board usually will not be below the waterline I do not shape it to any “Streamlined” shape but leave the top part of the board squared off, rectangular and flat on the sides. The flat upper section provides a better bearing surface for a leeboard or swing up center board and lets a dagger board fit flush inside a rectangular opening in a dagger board box.

The thin plywood can be “Tortured” to transition from the flat upper section to the “Streamlined”, underwater part.

If the top of the board needs a radius cut to clear the inside of a centerboard trunk the top of the board can be cut to shape after it is assembled, taking care not to hit any metal attachments that may be hiding inside.

The only real “Trick” is deciding how thick the long stringer needs to be and exactly where to place it to get a suitable “Foil” (used very Loosely again) shape. I usually make the stringer a little thicker than the nose piece and for the 12-16” boards, about 1-1/2” wide and about an inch shorter than the distance from the bottom of the spacer/reinforcing piece to the bottom end of the board.

When I get the stringer where I think it should be I mark the position on the inside of one side skin and then Measure that distance to mark the corresponding position on the inside of the other skin so the stringer will be equal distance from the front of the board.

I haven’t found it necessary to round over the sides of the stringer on these smaller boards, but on a larger board it should be rounded to match the inside curve of the plywood skin to prevent any “Hard Spots” in the skins.

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I guess if one did all the math one could cut the nose piece and stinger(s) to a size that would produce a real “Foil” shape.

As I said, I guess if one did all the math one could cut the nose piece and stinger(s) to a size that would produce a real “Foil” shape. That might take more than one stringer and maybe a couple of ribs, but I’m just looking for something that Looks good.

Locating the position of the stringer by trial and error has worked well enough for me. I pick a place a little less than half way from the leading edge then clamp the skins around that and see how it looks until I’m happy.

For the first, “Elegant Punt” leeboard I didn’t make a “Trailing edge piece” but just glued the skins together at the trailing edge. On the next board I built, I cut a small, thin, triangular piece of wood with rabbets similar to the leading edge piece to fit between the back edges of the plywood and that seemed to make a neater, sharper “Trailing edge”.

When I get the Leading edge piece, the skins, the trailing edge piece and the internal blocking at the top cut and placed where I think they should go, I use some temporary screws and a few clamps to hold the whole thing together so I can get a good look. That’s the time I trace around the bottom so I can make a solid wood “Bottom End Piece”. That piece is cut to the same “Streamlined” shape of the board and a rabbet is cut around the top so it will fit inside the skins.

The reason the nosepiece, trailing edge piece and the stringer(s) need to be slightly shorter than the skins is so they won’t be in the way of the “Bottom End Piece” when it is fitted in place. Cutting and fitting the bottom end piece is probably the most difficult part of the whole job.

Before final assembly I coat all internal parts and insides of the skins with epoxy and let the epoxy dry. The board May be water tight, but just in case, I want the insides protected. I converted one board built for use on another boat and had to drill some additional holes. I found water dripping out one of the earlier holes I had forgotten to plug, so was glad I had coated everything inside with epoxy, just in case something like that happens.

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Before final assembly be Sure to round over any places that will be difficult to get to after it’s all put together.

Now’s the time you need to calculate how much internal ballast you might need or want and make accommodations for securing that inside the bottom end of the board. A couple of screws into the bottom pieces to give the lead something to hold on too and some thickened epoxy should hold some lead in place OK.

Before final assembly be Sure to round over any places that will be difficult to get to after it’s all put together.

If everything was dry fitted and held together with temporary screws, final assembly should go pretty smoothly. Just slather all the joining parts with slightly thickened epoxy, put everything where it’s supposed to go (Be careful to make sure stringer is properly aligned) and put the temporary screws back in where they were.

After the epoxy sets you can take out all the temporary screws, ready to be used on the next project. I just learned about using hex head sheet metal screws for temps. The heads don’t strip out like sheet rock screws and they can be used over and over.

I use a large round over router bit to round over the leading edge piece and the edges of the “Bottom End Piece”. Just a part of a very large radius bit makes a good, “Leading Edge” shape. Also round over any parts you think might need it, like lifting handle. Drill a pivot hole if needed. I like to drill an oversized hole for the pivot pin and put in some sort of “Bearing”. Fill any joints between plywood and solid pieces if necessary and sand to suit. Be sure to paint, or if you want the board finished Bright, be sure to cover epoxy with varnish or some other covering to protect the epoxy from UV.

I don’t know if it’s necessary on smaller boards, but I glassed the entire outsides of the boards I’ve built to protect them and add to strength. If you chose not to glass the whole thing, it might be good to at least glass the leading edge and bottom end piece as they may take a beating.

The process sounds more complicated than it really is. It Is Fun to build a hollow board and when you get done you’ll have a neat, very strong board with that “Streamlined Shape” we are always looking for. As I’ve found, you will also have a “Conversation piece” and you may pass on this method to others.

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I can bridge the board between two points at the ends, stand on the board and it doesn’t even bend.

My 4 Foot long “Elegant Punt” board is Way over built. I can bridge the board between two points at the ends, stand on the board and it doesn’t even bend. If I hit something solid with this board, the side of the boat is going to tear off befor this board breaks. I built the one shown here that is about the same size as the “Elegant Punt” board and is more in proportion to the 11’ skiff I use it on.

This same technique can be used to build larger boards and large rudder blades too. I think this method would have real advantages for Large underwater boards. The skins for an 8’x 32” leeboard for one of Bolger’s larger boats could be made from a single sheet of 4 X 8 plywood cut on an angle. A couple of pieces of solid wood for the leading edge, top blocking, bottom piece and stringer(s) and you could have a pretty big, strong, cheap board. Compare the cost of those parts to the cost of materials for any solid board, be it plywood or lamented solid stock.

I had planned to take some photos showing the start to finish construction of my latest hollow leeboard to use with a “Michalak Style” mounting, but I was having so much Fun and everything went so quickly, I forgot to get any pics until the board was nearly finished. But here are a couple of pics of the nearly finished board. Maybe you can see enough detail to be helpful.

Please let me know if anybody Has built a hollow board like this or if you Do build a hollow board. All comments are welcome, but Please No. “That’s not a ‘Real “Foil’ “. I Know that. <G>

It sure Looks Cool and Works Great.

I think these hollow boards have a lot of advantages and I will use them on Any sailboats I build from here out.

The technique is pretty easy and I hope self explanatory, but if you do have any questions or Ideas, please contact me at pateson@colton.com (Only One “t” in That pateson )

Have Fun

Pat Patteson

Molalla, Oregon